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How does the body respond to stress?

Updated on April 10, 2012
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Selye (1950) defined stress as "the nonspecific response of the body to any demand", which basically means it is a generalised reaction to demands, or 'stressors'. When we are exposed to a stressor, it arouses a stress reponse which is useful in situations where our survival is threatened (think of the 'fight or flight' response and when that is useful!). The stress response is an innate, defensive and adaptive mechanism.

Now, when we experience a stressor, our autonomic nervous system is involved in the bodily response, so it will be useful to know just what that is. Our autonomic nervous system (ANS), is divided into two parts: the central nervous system (which involves the brain and spinal cord), and the peripheral nervous system, (which involves all the other cells in the body). The peripheral nervous system futher divides into: the somatic nervous system (which is concerned with voluntary movements of skeletal muscles, and the autonomic system (concerned with involuntary movements of non skeletal muscles i.e. the heart).

The ANS is largely an automatic and self regulating sysyem, which responds with little or no conscious thought. It is concerned with many vital functions such as breathing and digestion. The ANS has two functions: to activate internal organs, and to save energy. So, it has two branches for this, (I hope you're keeping up!). These are the sympathetic, and parasympathetic branches. The sympathetic branch activates the interal organs in situations where you need energy and arousal; it is responsible for the fight or flight response, and produces an increased heart rate, reduced activity in the stomach, pupil dilation or expansion, and relaxation of the bronchi in the lungs. On the other hand, the parasympathetic branch is responsible for conserving and storing resources; it monitors the relaxed state and promotes digestion and metabolism. It produces opposite effects to the sympathtic nervous system. The two branches often operate antagonistcally; for example, the heart rate will be high if there is more sympathetic branch activity, and low if there is more parasympathetic branch activity. However, they sometimes work together!!

Firstly, in case you're wondering just how the ANS achieves its effects, well, it's down to the endocrine system. The endocrine system consists of various ductless glands which secrete hormones into the bloodstream, which then control the ANS activity. Hormones can have dramatic effects on our behaviour and emotions. Most are slow acting, and this is because the bloodstream carries them around the body relatively slowly. The effects of these hormones can last for some time, but typically gradually dimish as a situation becomes less stressful.

Homeostatis is a result of ANS activity which is a fundamental part of the stress reponse. When an individual is placed under stress, the body reacts, but wants to return to a normal state as soon as possible! The normal state is controlled by the parasympathetic branch. It strives for a balance between sympathetic and parasympathetic activity, and stress is often experienced when this balance cannot be achieved.

So, what do we have so far? We know that the autonomic nervous system controls the body's response to stress, and we know that it splits and splits until it finally gets down to two branches, the sympathetic branch and the parasympathetic branch. The sympathetic branch produces arousal for the fight or flight response, and the parasympathetic branch calms it down again in order to reach homeostatis (a steady state). Now, let's continue even further - it gets a little more complex than that!

There are two major bodily responses to stress, one being the short term, instant response, to acute stress, and the other being a longer term response to chronic stress. We call the initial response to shock the 'sympathetic adrenal medullary system', or simply 'SAM'. The activity in the sympathetic branch of the ANS stimulates the adrenal medulla, which forms a part of your adrenal glands. When stimulated, the adrenal meddula secretes the hormones adrenaline, and noradrenaline. These hormones increase the output of the heart, which in turn increases your blood pressure. They lead to increased arousal of the sympathetic nervous system and reduced activity in the parasympathetic branch, so, we know from earlier that at this point your heart rate will be increasing and other bodily responses will be occurring. Heightened activity of the SAM is what prepares us for the fight or flight response. The SAM response causes an increase in energy, alertness, blood flow to the muscles, heart and respiration rate, and clotting factors in the blood (in case of injury), along with reduced activity in the digestive system (explaining why many of us lose our appetite when we are stressed!). The SNS plus the SAM = sympathomedullary pathway. The issue with the SAM response is that it is not only associated with stress; elevated levels of adrenaline and noradrenaline can be experienced when we are concentrating hard on a task. The issue is how we percieve our internal physiological state. Sometimes, we perceive heightened SAM activity as indicating that we are stressed, and sometimes we interpret it as meaning we are excited or stimulated.

The long term response to stress is called the 'hypothalamic-pituitary-adrenocortical axis', or simply 'HPA'. If we are exposed to stress for several hours or more, the SAM response is increasingly using up our bodily resources - this can't go on forever, which is why we have the countershock response, HPA, designed to minimise any damage caused. Most of the glands of the endocrine system distributed throughout the body are controlled by the hypothalamus which produces hormones that stimulate the anterior pituitary gland. The most important hormone is the adrenocorticotrophic hormone (ACTH). The key glucocorticoid with respect to stress is cortisol (excessive amount are found in the urine of stressed people). ACTH stimulates the adrenal cortex, which produces various glucocorticoids which are hormones that have an effect on glucose metabolism. It is important to note that the HPA response is much harder to ahcieve, and is only for chronic situations. The good effects of the HPA response include that cortisol is actually very important in coping with long term stress as it provides a steady supply of fuel and reverses some of the body's initial response to stress. The bad effects include that the anti-inflammatory action of glucocorticoids slows wound healings and suppresses the immune system.

Selye (1936, 1950), who we met earlier with his definition of stress, devised something called the 'general adaptation system', or GAS. In 1936, Selye wrote his first article on the effects of stress. He exposed rats to nocuous agents (harmful), and noted that the same symptoms appeared in response to all of the stimuli; he concluded that all symptoms were due to a general state - stress. Selye then argued that stress is adaptive in the short term as it enables us to cope with environmental demands. However, our body's reaction to long term stress can be damaging. The beneficial effects of HPA can't continue indefinitely at an elevated level of activity, and if the adrenal cortex stops producing glucocorticoids it eliminates our ability to maintain blood glucose concentrations at appropriate levels. Selye noticed that rats and hospital patients showed a similar pattern of bodily response, which is why he called it the general adpatation system. It represents the body's attempt to cope adaptively with stress. He argued that GAS consisted of three stages:

1. Alarm reaction: Increased activity in SAM and HPA. According the Selye, the alarm reaction develops 6-48 hours after stress (e.g. injury), and it includes a loss of muscular tone, a drop in body temperature, and a decrease in the size of the liver. It emphasises HPA.

2. Resistance: This is the stage of adaptation, and also involves HPA activity. It is the body adapting to the demands of the environment. As the stage proceeds, the parasympathetic nervous system requires more careful use of the body's resources in order to cope. The system is being taxed to it's limits.

3. Exhaustion: When stress is very prolonged, the physiological systems used in the previous two stages eventually become ineffective. The initial ANS symptoms of aroousal re-appear: increased heart rate, sweating etc. In extreme cases, the damaged adrenal cortex leads to the failure of the PNS (metablism and storage of energy) and a collapse of the body's immune system. Here, stress related diseases are more likely.

Evaluation of GAS: It correctly focused on the HPA response, and the importance of glucorticoids. It also alerted medicine to the importance of stress in disease, meaning more research could go into it to help those most at risk. However, it didn't pay much attention to the SAM system, and didn't fully understand the relationship between SAM and HPA. Selye also exaggerated when he claimed that stress always produces the same physiological pattern, as seen in the following study supporting the opposite claim.

Mason (1975) - He compared reactions to stressors, varying in how much fear, anger or uncertainty they created. The various stressors produced different patterns of adrenaline, noradrenaline and cortisol secretion.

Seyle has been criticised for using non human animals to support his research on human responses to stress, and it may explain why the model exaggerates the importance of physiological factors at the expense of psychological factors like the role of emotional and cognitive factors.

Finally, Seyle was criticised by Symington et all (1955) for assuming that people respond in a passive way to stressors. Symington studied two groups of dying patients, one group who were in comas, and the other concious. He found that there were more signs of physiological stress in the concious patients, indicating that they engaged in stressful psychological appraisal of their state.


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